TRANSLATIONAL DIFFUSION IN SUCROSE SOLUTIONS IN THE VICINITY OF THEIRGLASS-TRANSITION TEMPERATURE

The prediction of the stability of low-moisture products is complex, p articularly close to the glass transition temperature, This study demo nstrates that the relations used to evaluate the influence of temperat ure on the viscosity of carbohydrate media cannot be applied to the di ffusivity. The translational diffusion coefficient of a fluorescent mo lecule (fluorescein) is measured in sucrose-water mixtures as a functi on of temperature, The main result is that the mobility of the fluores cein is not simply coupled to the viscosity of the diffusion medium at temperatures close to the glass transition temperature. Indeed the WL F equation, which gives a good prediction of the viscosity, does not a llow the determination of the diffusivity in a temperature range close to T-g because the translational diffusion follows a weaker temperatu re dependence. Different possible explanations for this apparent decou pling between translational diffusion and viscosity are suggested: a s mall change in the hydrodynamic radius of the diffusing molecule due t o low water content and/or a connection with the beta relaxation proce ss.